Power control is important in mobile telephone networks, for example, because it is important to obtain desirably high capacity and efficiency, particularly in CDMA systems. The variable that is controlled is called quality. Quality of the communication is controlled with reference to a quality measure such as: BER (Bit Error Rate), FER (Frame Erasure Rate, BLER (Block Error Rate), number of iterations of a turbo decoder, or the average reliability of decision statistics. Below, for the sake of brevity, the quality measurement will be referred to as BLER. It will, however, be readily appreciated that BER or FER, or other quality measurement could be used.
Usually an integrating controller is provided to achieve a steady state performance with zero control error. The control scheme used is cascade control, see for example FIG. 1 of the accompanying drawings. The idea with cascade control is to make an inner control loop (2) much faster than an outer control loop (4). For transmission power control (TPC) the inner loop controls another quality measure such as for example the signal to interference ratio (SIR). The outer loop sets the SIR reference value SIRr for the inner loop. The goal of the outer loop is to control the SIR reference value to achieve a BLER that is equal to the BLER reference BLERr. To get a control system that in steady state achieves a BLER that is equal to the BLER reference, an integrating controller (9), which can be, for example, a PI controller, a PID controller, or a pure integrating controller, can be used. The cascade controller illustrated in FIG. 1 comprises an inner control loop (2) and an outer control loop (4). Both control loops have an input of a received signal (y(k)). In the outer control loop 4, the BLER is estimated in a BLER estimation unit (5) and compared with a BLER reference signal. A subtractor (7) calculates the difference between the reference signal and the BLER estimate to supply an input signal to an integrating controller (9). The integrating controller (9) produces a SIR reference signal.
The SIR reference signal is compared with an SIR estimate from an SIR estimation unit (3) in the inner control loop (2). The difference between the SIR reference and the SIR estimate is supplied to a function, for example a step function (11) for determining a command u(k) that sets transmission power. More generally, the SIR estimate and the SIR reference value could both be supplied to a function that determines a command u(k) for setting the transmission power.
A known problem with an integrating controller (such as a PI, PID, or pure integrating controller) is that it becomes unstable if the control signal saturates. This problem is often referred to as the windup problem. Transmission power control (TPC) saturation of the control signal corresponds to situations when the maximum (or minimum) transmitter power is used.
The windup problem in the power control algorithms for third generation mobile telephony systems is well known. The specific problem of windup protection in WCDMA makes several additions to anti-windup schemes used in other areas necessary.
As is well known, integrating controllers have the nice property of being able to achieve zero control error in steady state. As an example of an integrating controller, a continuous time PI-controller is shown in FIG. 2. Discrete time controllers have similar behaviour; see, for example, Karl Johan {dot over (A)}ström and Tore Hägglund, “PID Controllers: Theory, Design and Tuning”, Instrument Society of America, Research Triangle Park, N.C., second edition, 1995.
A known problem with integrating controllers is that the integrator part turns unstable when the control signal saturates. This instability occurs because feedback from the process is needed to stabilize the controller, which is not open loop stable. In the case of transmission power control, saturation can occur when maximum (or minimum) transmission power is used. In this situation the transmission power can only be decreased (or increased in the case of a minimum), which can be seen as open loop operation of the integrator.
As the controller is not open loop stable the controller state (the integrator, I-part) can start to build up a large state. This usually results in that it takes a long time for the control loop to start functioning again after the saturation state is left. This problem is usually referred to as the windup problem.